The invention pertains to the field of communications interfaces via which DC power is provided to operating circuitry.
Powered communications interfaces are utilized in data communications systems to provide operating power to devices over the same wires used to carry data communications signals, in a manner analogous to the traditional telephone system in which DC operating power is provided to subscriber equipment over the twisted-pair telephone wires. Today, there is widespread use of so-called “power over Ethernet” or POE technology, in which DC operating power is provided to digital telephones, video cameras, and other data terminal equipment over unshielded twisted pair (UTP) cables connecting the data equipment with centralized data switches. In POE parlance, a device receiving power in this fashion is termed a “powered device” or PD, while a device that provides power for use by PDs is termed a “power sourcing equipment” or PSE.
According to applicable POE standards, a PSE must detect and classify a PD before PSE power is delivered to the PD. The PD presents a 25 kΩ signature resistor (R-signature) to a PSE to request the delivery of POE power. During the detection process, a PSE successively applies relatively low voltages V1 and V2 (less than 30 V) while measuring corresponding currents I1 and I2 conducted by the PD, then it calculates a resistance value R-signature=(V2−V1)/(I2−I1). If this calculation yields an R-signature in a suitable range about 25 kΩ (the valid identity network for a PD requesting power), the PSE proceeds to a classification process to ascertain the power requirements of the PD. The PSE applies a voltage in the range of 15 v-20 v while measuring the current drawn by the PD, and then uses the current value to classify the PD according to a set of values specified in the standard. Traditionally the standard allows 5 classes (labeled 0 to 4), and a more recent version of the standard allows for additional devices that require higher power than previously defined. Other PD detection techniques are also known. For example, PD detection may also be carried out using differential loopback of a communications signal from a PD using two wire pairs, or using a single-pair identity network and appropriate signaling and detection.
Conventionally, once detection and classification are complete, a PSE automatically applies full power (48 volts and a class-based maximum current) to the PD via the powered communication interface as long as the PSE has sufficient incremental power available to do so. The PD uses this POE power to operate. In many cases, the 48 V power is supplied to one or more DC-DC converters in the PD which transform the 48 V power into other specific operating voltages as required by the PD operating circuitry, such as +/−15 V, +3.3 V, etc. In particular, the 48 V power is used to provide power to communications circuitry within the PD that effects high-speed data communications to/from the PD over the same twisted pairs used to carry the POE power. This circuitry is commonly referred to by the term PHY, referring to its “physical layer” communications functionality according to the well-known hierarchical description of data network communications.